I have used all 8 of the lores one way or another, here are some of my favourites
L4, lore of beasts on a griffon, mainly because the model is awesome, and his presence confuses my opponent, but amber magic is awesome if used correctly as it can increase the strength of our troops, boost our characters and the griffon gives him great manouvreability as well as a punch in combat.
L4 lore of metal, metal synergises very well with Empire and has a wide range of spells for every occasion.
L2/1 lores of heavens, shadow or beasts; you bring these guys for the sig spells of their respective lores
Shadow and life are great lores, but they don't really fit with the way I play my Empire force so I tend not to use them, personally I feel life doesn't work particularly well unless you have the loremaster rule because throne is so essential and the chance of getting 2 or more doubles on your spell selection roll is pretty remote.